Metal-free CVD coating of graphene on glass and other dielectric substrates

ABSTRACT

A catalyst-free CVD method for forming graphene. The method involves placing a substrate within a reaction chamber, heating the substrate to a temperature between 600° C. and 1100° C., and introducing a carbon precursor into the chamber to form a graphene layer on a surface of the substrate. The method does not use plasma or a metal catalyst to form the graphene.

BACKGROUND

Field

The present disclosure relates generally methods for forming graphenethin films, and more specifically to a CVD route for directly forminggraphene layers directly on dielectric substrates.

Technical Background

Graphene is a two-dimensional allotrope of carbon. The structure ofgraphene includes a single planar sheet of sp²-hybridized carbon atomsarranged in a densely-packed honeycomb array. The carbon-carbon bondlength in graphene is about 0.142 nm. A schematic of a graphenemonolayer is shown in FIG. 1.

In essence, graphene is an isolated atomic plane of graphite. As a2-dimensional crystalline material, graphene has unique propertiesincluding high intrinsic mobility (200,000 cm²V⁻¹s⁻¹), Young's modulus(˜1,100 GPa), breaking strength (42 Nm⁻¹), fracture strength (˜125 GPa),thermal conductivity (˜5000 Wm⁻¹K⁻¹), surface area (2,630 m²g⁻¹), and anoptical transmittance of ˜97%. With such remarkable properties, graphenehas a wide variety of potential applications that range fromnano-electromechanical resonators and high-performance field effecttransistors to clean energy devices, sensors and antibacterial products.

Graphene was first isolated via mechanical exfoliation ofhighly-oriented pyrolytic graphite (HOPG). It is now well-known thattiny fragments of graphene sheets are produced whenever graphite isabraded, such as when drawing with a pencil. Graphene can also beobtained via carbon segregation by heating a carbon source such assilicon carbide to high temperature (>1100° C.) at low pressure (˜10⁻⁶Torr) to reduce it to graphene.

The lack of a large-scale synthesis route for the production ofhigh-quality graphene at low cost has substantially hampered itsproliferation. Accordingly, it would be advantageous to develop aneconomical method for forming large area graphene.

BRIEF SUMMARY

In accordance with embodiments of the present disclosure, a method forforming graphene comprises placing a substrate within a reactionchamber, heating the substrate to a temperature between 600° C. and1100° C., introducing a carbon precursor into the chamber and forming agraphene layer on a surface of the substrate, wherein the substrate isfree of a metal catalyst and the chamber is free of plasma during theforming.

A graphene-coated substrate comprises a dielectric substrate such as aglass, ceramic or glass-ceramic substrate, and a graphene layer formedin direct contact with a surface of the dielectric substrate.

Additional features and advantages of the subject matter of the presentdisclosure will be set forth in the detailed description which follows,and in part will be readily apparent to those skilled in the art fromthat description or recognized by practicing the subject matter of thepresent disclosure as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the subjectmatter of the present disclosure, and are intended to provide anoverview or framework for understanding the nature and character of thesubject matter of the present disclosure as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe subject matter of the present disclosure, and are incorporated intoand constitute a part of this specification. The drawings illustratevarious embodiments of the subject matter of the present disclosure andtogether with the description serve to explain the principles andoperations of the subject matter of the present disclosure.Additionally, the drawings and descriptions are meant to be merelyillustrative, and are not intended to limit the scope of the claims inany manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of specific embodiments of thepresent disclosure can be best understood when read in conjunction withthe following drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 is a schematic diagram of graphene according to one embodiment;

FIG. 2 shows Raman spectra of graphene layers formed on silicasubstrates;

FIG. 3 is a plot of graphene grain size versus acetylene exposure time;

FIG. 4 is a plot of Raman G/2D intensity ratios versus total acetyleneexposure;

FIG. 5 shows Raman spectra for graphene layers formed on silica (SiO₂)substrates at 1000° C. under different conditions of pressure and growthtime;

FIGS. 6a and 6b are atomic force microscope (AFM) images of a graphenelayer;

FIG. 7 shows Raman spectra for graphene formed on different glasssubstrates;

FIG. 8 shows Raman spectra for graphene formed on Eagle XG® glass andsilica glass;

FIG. 9 is a plot of transmittance versus wavelength for graphene layersaccording to embodiments;

FIG. 10 is a plot of Van der Pauw sheet resistance for differentgraphene layers;

FIG. 11 is a plot of graphene sheet resistance versus transmittance at550 nm;

FIG. 12 is a plot of Hall mobility for graphene layers on various glasssubstrates;

FIG. 13 shows Raman spectra for graphene formed on a cordieritehoneycomb substrate;

FIG. 14 is an SEM micrograph of a graphene-coated cordieritecrystallites according to embodiments;

FIG. 15 is an SEM micrograph of a graphene layer formed on cordieriteaccording to embodiments; and

FIG. 16 is an SEM micrograph of wrinkles formed in a graphene layeraccording to embodiments.

DETAILED DESCRIPTION

Reference will now be made in greater detail to various embodiments ofthe subject matter of the present disclosure, some embodiments of whichare illustrated in the accompanying drawings. The same referencenumerals will be used throughout the drawings to refer to the same orsimilar parts.

Direct CVD growth of graphene on dielectric substrates is disclosed. Themethod is performed without plasma or a metal catalyst. According toembodiments, a graphene-coated substrate comprises a graphene layerformed in direct contact with a surface of a dielectric substrate. Thesubstrate can comprise glass, ceramic or glass-ceramic.

In contrast to conventional layer transfer methods, the instant CVDapproach involves fewer processing steps, which minimizes the potentialfor contamination of or damage to the graphene layer orgraphene-substrate interface. For example, the graphene-substrateinterface can be free of molecular adsorbates such as water and hydroxylgroups. In embodiments, the concentration of such molecular adsorbatesat the graphene-substrate interface is less than 0.5 at.%. The presentCVD method may be used to form well-adhered graphene layers on planarand non-planar substrates including substrates have concave or convexfeatures such as porous substrates and honeycomb substrates.

Graphene is essentially a one-atom thick layer of graphite. In thegraphene structure, carbon atoms are covalently bonded to each other. Asillustrated in FIG. 1, the plurality of carbon atoms may formsix-membered rings as the typical repeating unit. Graphene may furthercomprise 5-membered and/or 7-membered rings. Thus, graphene manifests asa single layer or plural layers of covalently bonded (sp² hybridized)carbon atoms. The graphene layer thickness may range from about 0.34 nm(monolayer) to 100 nm (plural layers). In embodiments, a graphene layermay include up to 100 atomic layers of carbon. Thus, in addition tomonolayer, bi-layer and tri-layer graphene, a graphene layer maycomprise N atomic layers of carbon where 4≦N≦100.

Graphene layer(s) are formed via thermal decomposition of a carbonprecursor. Example carbon precursors include acetylene, ethylene andmethane. Acetylene, which has a comparatively low dissociationtemperature, was used in several experiments to deposit graphene onnon-metal-catalyzed substrates without the use of plasma. Inembodiments, the substrate is free of a metal catalyst such as copper ornickel, which are used conventionally to induce graphene formation. Inembodiments, no metal catalyst is in physical contact with the substrateduring graphene deposition on the substrate.

In an example process, a substrate is placed within a reaction chamberand acetylene (or another carbon precursor) is introduced into thechamber at a specified chamber pressure while the substrate is heated toa specified substrate temperature. The chamber pressure may range from0.001 Torr to 760 Torr (e.g., 0.001, 0.002, 0.005, 0.01, 0.02, 0.05,0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, 100, 200, 500 or 760 Torr, includingranges between any of the foregoing values), and the substratetemperature during the deposition may range from 600° C. to 1100° C.(e.g., 600, 700, 800, 900, 100 or 1100° C., including ranges between anyof the foregoing values). Substrates may include dielectric substratessuch as glass, ceramic and glass-ceramic materials. Using the disclosedCVD process enables direct deposition of graphene on many differentglass substrates. Example substrates include Eagle XG® glass, CorningWillow™ glass, and Corning Lotus™ glass. In contrast to PECVD, which isrestricted to near line-of-sight layer growth due to shadowing andplasma penetration limitations, thermal CVD can be used to form graphenelayer(s) within the pores or channels of a substrate.

The graphene layers may be characterized by an average thickness rangingfrom 5 to 50 nm and an average grain size ranging from 10 to 100 nm,e.g., 15 to 40 nm or 20 to 30 nm Optionally, the average grain size ofthe CVD deposited graphene layers can be increased using apost-deposition heat treatment such as a vacuum heat treatment. Apost-deposition heat treatment can increase one or both of theconductivity and transparency of the graphene.

EXAMPLES Example 1 Graphene Layers Formed on Silica Glass

Fused SiO₂ glass substrates were cut into coupons measuring 1 in×0.75in. The individual coupons were cleaned by first immersing for 3 min in5N NH₄OH, and then rinsing alternately with 18 MΩ water and methanol.After rinsing, the silica pieces were inserted into a 1 in diameter tubefurnace. The reaction chamber was evacuated using a mechanical pump to abase pressure of 5×10⁻² Torr. The substrate temperature was increased tothe deposition temperature and acetylene gas was introduced to thereaction chamber at a prescribed pressure.

Raman spectroscopy was used to characterize the resulting graphenelayers. Raman spectra of graphene layers formed on silica (SiO₂)substrates at substrate temperatures ranging from 600° C. to 1100° C.are shown in FIG. 2. The respective D, G and 2D graphene peaks areindicated.

The G band (˜1580 cm⁻¹) and 2D band (˜2700 cm⁻¹) are characteristicfeatures of graphene. The G band is due to in-plane resonance fromgraphitic sp² bonding. Without wishing to be bound by theory, theintensity of the G band peak is proportional to the amount of graphiticstructure within the graphene. The 2D band originates from a two phonondouble resonance Raman process, and is closely related to the bandstructure of graphene layers. The D band provides an indication of theamount of defects within the graphene layer(s).

From the intensity ratio of the D/G bands, the size of the graphenegrains can be determined from the equation d=(2.4×10⁻¹⁰)λ⁴/(I_(D)/I_(G)), where λ=514 nm. A plot of grain size versus acetyleneexposure time for various growth temperatures is shown in FIG. 3. InFIG. 3, the graphene grain size ranges from about 17 nm to about 27 nm,which corresponds to respective D/G ratios of 0.99 and 0.62. The D/Gratio may range from 0.6 to 1.5 (e.g., 0.6, 0.7, 0.8, 0.9, 1.0, 1.1,1.2, 1.3, 1.4 or 1.5, including ranges between any of the foregoingvalues).

The intensity of the 2D band is inversely proportional to the thicknessof the graphitic layers. Monolayer graphene is characterized by a G/2Dratio of less than 1. Bi-layer graphene is characterized by a G/2D ratioof unity. Multilayer graphene is characterized by a G/2D ratio greaterthan 2.

FIG. 4 is a plot of Raman intensity ratio (G/2D) versus acetyleneexposure (pressure×time) for various growth temperatures. Generally, thehigher the deposition (substrate) temperature, the flatter the graphenegrains. In FIG. 4, the minimum G/2D intensity ratio is 2.1, indicatingthat the graphene contains only a few layers of carbon.

Shown in FIG. 5 are Raman spectra for graphene layers formed on silica(SiO₂) substrates at 1000° C. for reaction chamber pressures rangingfrom 0.1 to 1 Torr and growth times ranging from 30 to 60 min. The Dband intensity decreases and the 2D band intensity increases withincreased deposition time, consistent with larger and flatter grains. Asa function of deposition pressure, the 2D band intensity exhibits amaximum at intermediate pressures (˜0.5 Torr) and decreases at bothlower (0.1 Torr) and higher (1 Torr) pressure.

FIG. 6a is an atomic force microscope (AFM) image of a graphene layer ona silica substrate. FIG. 6b is an AFM image of the graphene layerpartially delaminated from the substrate using Scotch tape. A heightprofile is also shown in FIG. 6b . The graphene layer was formed at asubstrate temperature of 1000° C. The data shown in FIGS. 6a and 6bcorrespond to the second trace from FIG. 5 (0.5 Torr acetylene; 60min.). The graphene layer thickness is 10-20 nm.

Example 2 Graphene Layers Formed on Different Glass Substrates

A high temperature glass containing SiO₂, Al₂O₃, MgO and CaO (ARV glass)with the composition shown in the Table 1 was ground into glass frit,and pressed into pellets. The pellets were sintered at 1000° C. for 2hours and used as substrates for direct graphene growth. A comparison ofthe graphene layers formed on the ARV glass was made to graphene grownat the same condition on SiO₂ glass, a comparative Ni-coated SiO₂ glass,and a comparative Ni-containing glass (AXZ glass) substrate.

TABLE 1 ARV and AXZ glass substrate compositions (mole %) ARV AXZ SiO251.9% 49.1% Al2O3 22.8% 21.6% MgO 18.2% 17.2% CaO 7.1% 6.8% NiO 4.7%

Raman spectra of comparative graphene layers formed on catalyzedsubstrates and inventive graphene layers formed on non-catalyzedsubstrates at 1000° C. in 0.5 Torr acetylene for 1 h are shown in FIG.7. The substrates include (a) nickel coated silica glass, (b)nickel-containing glass (AZX), (c) silica glass, and (d) nickel-freeglass (ARV). Graphene layers formed on the nickel-coated silica glassand the nickel-containing glass are comparative. The presence of Ni,either as a thin layer on the deposition surface or incorporated intothe glass composition, enhances the graphene grain size (lower intensityD band) and flattens the grain shape (higher intensity 2D band). Thequality of the graphene layers formed on ARV glass was similar to thatof the graphene formed on SiO₂ glass.

FIG. 8 shows similar Raman spectra for graphene layers deposited via CVDat 700° C. in 10 Torr acetylene for 1 hr onto a Corning Eagle XG® glasssubstrate and onto a silica glass substrate.

Example 3 Electrical and Optical Properties of Graphene

Transmittance spectra for graphene layers formed via acetylene CVD onsilica (SiO₂) substrates at 1100° C. using a reaction chamber pressureof 0.5 Torr for growth times of 5, 12 and 30 min are shown in FIG. 9. At550 nm, as the growth time and therefore the layer thickness increases,the transmittance decreases from 94.46% to 75.69% to 63.58%,respectively. Van der Pauw sheet resistance data are plotted in FIG. 10for graphene layers formed via acetylene CVD at 1100° C. using areaction chamber pressure of 0.5 Torr for various growth times. FIG. 11is a plot of sheet resistance versus transmittance at 550 nm. This dataindicate that the graphene layers deposited at 900-1100° C. are similarin quality, while layers formed at lower temperatures have a lowerquality. FIG. 12 is a plot of Hall mobility data for graphene layers onvarious glass substrates. The Hall mobility ranges from 1 to 300 cm²/Vs.Higher mobility is associated with a higher deposition temperature.

Example 4 Post-deposition Graphene Refinement

Several post-deposition approaches were explored with the goal ofimproving the quality of the as-deposited layers. In embodiments, apost-CVD vacuum heat treatment was used. The heat treatment wasconducted within the reaction chamber after stopping the flow of carbonprecursor.

The heat treatment temperature may range from 900° C. to 1200° C., e.g.,900, 1000, 1100 or 1200° C. for 0.25, 0.5 or 1 hr. As an example, agraphene on silica glass sample, which was deposited at 1000° C. in 0.5Torr acetylene for 30 min, was heated at 1000° C. in vacuum (5×10⁻⁶torr) for 1 hr. The Raman-measured graphene grain size increased nearly12%, from 22.5 nm to 25.1 nm.

In embodiments, the as-deposited graphene can be covered with a metalfoil or a metal-coated substrate such that the metal is in contact withthe graphene during the post-deposition heat treatment. Example metalsinclude nickel or copper.

A graphene layer was formed on a silica substrate at 1000° C. in 0.5Torr acetylene for 90 min. After the deposition, the graphene was heatedat 1000° C. in vacuum (5×10⁻⁶ torr) for 1 hr during which time thegraphene surface was cover by a Ni-coated (200 nm) SiO₂ glass, a Ni foilor a nickel-containing glass. The post-deposition heat treatment usingthe nickel-coated glass in contact with the graphene increased thegraphene grain size from 25.52 nm to 26.88 nm (5.3%) and thepost-deposition heat treatment using nickel foil in contact with thegraphene increased the graphene grain size from 25.52 nm to 26.73 nm(4.7%).

Example 5 Remote Graphene Catalysis

In embodiments, a metal foil, metal-coated substrate or metal-containingsubstrate can be placed proximate to the dielectric substrate such thatthe metal is not in physical contact with the graphene or the dielectricsubstrate during graphene growth. By proximate is meant that the metalfoil, metal-coated substrate or metal-containing substrate is at adistance of less than 5 mm (e.g., about 0.5, 1, 2 or 5 mm) from thedielectric substrate.

A graphene layer was formed on a silica substrate at 900° C. in 0.5 Torracetylene for 90 min. During the graphene growth, a nickel-containingglass (AXZ glass) was placed proximate to the silica substrate. Thegraphene grain size on a silica substrate without the remote catalystwas 21.75 nm, while the graphene grain size using the remotenickel-containing glass was 35.12 nm.

Example 6 CVD-derived Graphene on Cordierite Honeycomb Substrates

Graphene was formed on a cordierite honeycomb substrate (cell density400 cpsi, wall thickness 4 mil). Example deposition conditions were1000° C. in 0.5 Torr C₂H₂ for 1 hr. Graphene was formed throughout thehoneycomb channels and within the pores of the channel walls, which wasevidenced by the uniform color change of the honeycomb substrate frompale yellow to black. Following graphene deposition, the entirehoneycomb substrate is conductive. The measured conductivity is equal tothe conductivity of a graphene layer on a planar glass substrate, whichis consistent with the formation of a continuous thin film. Ramanspectra taken from the exposed channel wall surface (A) and from afracture surface within a channel wall (B) are shown in FIG. 13. Nosignificant difference in the spectra were observed for the twolocations.

FIG. 14 is an SEM micrograph of graphene-coated cordierite. A pore 800in the cordierite structure is evident. In the fractured sample of FIG.15, adjacent to fracture surface 830, the graphene layer 810 and thecordierite surface 820 are visible. Wrinkles 815 in the graphene layer810 are shown in FIG. 16. In embodiments, the graphene coats thechannels and pores of the honeycomb such that both the channels and thepores of honeycomb remain open.

Graphene-coated dielectric substrates, including graphene-coatedhoneycomb substrates, can be used in a variety of applications,including as electrical and thermal conductive substrates, hydrogenstorage, ultracapacitor electrodes, electrical catalyst supports, etc.

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “carbon precursor” includes examples having twoor more such “carbon precursors” unless the context clearly indicatesotherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, examples include from the one particular value and/or to theother particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred. Any recited single or multiple featureor aspect in any one claim can be combined or permuted with any otherrecited feature or aspect in any other claim or claims.

It is also noted that recitations herein refer to a component being“configured” or “adapted to” function in a particular way. In thisrespect, such a component is “configured” or “adapted to” embody aparticular property, or function in a particular manner, where suchrecitations are structural recitations as opposed to recitations ofintended use. More specifically, the references herein to the manner inwhich a component is “configured” or “adapted to” denotes an existingphysical condition of the component and, as such, is to be taken as adefinite recitation of the structural characteristics of the component.

While various features, elements or steps of particular embodiments maybe disclosed using the transitional phrase “comprising,” it is to beunderstood that alternative embodiments, including those that may bedescribed using the transitional phrases “consisting” or “consistingessentially of,” are implied. Thus, for example, implied alternativeembodiments to a substrate comprising glass, ceramic or glass-ceramicinclude embodiments where a substrate consists of glass, ceramic orglass-ceramic and embodiments where a substrate consists essentially ofglass, ceramic or glass-ceramic.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Sincemodifications, combinations, sub-combinations and variations of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and their equivalents.

We claim:
 1. A method for forming graphene, comprising: placing asubstrate within a reaction chamber; heating the substrate to atemperature between 600° C. and 1100° C.; introducing a carbon precursorinto the chamber and forming a graphene layer on a surface of thesubstrate, wherein the substrate is free of a metal catalyst and thechamber is free of plasma during the forming, wherein the substratecomprises glass or glass-ceramic.
 2. The method according to claim 1,wherein the substrate is a honeycomb substrate.
 3. The method accordingto claim 1, wherein the substrate is heated to a temperature between700° C. and 1000° C.
 4. The method according to claim 1, wherein thecarbon precursor is selected from the group consisting of acetylene,ethylene and methane.
 5. The method according to claim 1, wherein thechamber pressure is from 0.001 to 760 Torr.
 6. The method according toclaim 1, wherein the graphene layer thickness is from 0.34 to 100 nm. 7.The method according to claim 1, wherein the reaction chamber is free ofhydrogen gas during the forming.
 8. The method according to claim 1,further comprising heating the graphene layer in vacuum without exposureto the carbon precursor.
 9. The method according to claim 8, wherein ametal catalyst is placed proximate to the graphene layer.
 10. A methodfor forming graphene, comprising: placing a honeycomb substrate within areaction chamber; heating the honeycomb substrate to a temperaturebetween 600° C. and 1100° C.; introducing a carbon precursor into thechamber and forming a graphene layer on a surface of the honeycombsubstrate, wherein the honeycomb substrate is free of a metal catalystand the chamber is free of plasma during the forming.